1. Field of the Invention
The present invention relates generally to an air-fuel ratio control system for adjusting the air-fuel ratio of an air-fuel mixture formed for an internal combustion engine to a predetermined or desired value and particularly to an air-fuel ratio control system of this type which is improved to stepwise or continuously reduce the effective cross sectional area of a passage, causing air to be drawn into a main fuel passage of a carburetor of the engine, with increases in the load of the engine to a desirable value at which the control range of the controlled air-fuel ratio is maintained at a desirable value.
2. Description of the Prior Art
As is well known in the art, an air-fuel ratio control system for an internal combustion engine is constructed and arranged to control the air-fuel ratio of an air-fuel mixture formed for the engine to a predetermined or desired air-fuel ratio by controlling, in accordance with the concentration of a component contained in exhaust gas of the engine which concentration is representative of the air-fuel ratio of an air-fuel mixture burned in the engine, the flow of air drawn into a main and/or slow fuel passage of a carburetor of the engine to adjust the flow the fuel drawn from the main and/or slow fuel passage into an intake passageway of the engine. It is necessary or desirable that the flow of the air drawn is so controlled that the control range of the controlled air-fuel ratio is maintained at a predetermined value independently of the load of the engine. However, in a conventional air-fuel ratio control system the flow of the air drawn into the main fuel passage has been undesireably or excessively increased with increases in the load of the engine so that the control range of the controlled air-fuel ratio has been increased to an undesirable or excessive value with increases in the engine load.